The present invention relates in general to electrified vehicles using a high voltage bus, and, more specifically, to accurate estimation of the effective isolation resistance present between each high-power bus and a chassis ground.
Electrified vehicles such as for electric vehicles and hybrid electric vehicles typically utilize a high voltage power bus driven by a DC power source which may include storage and/or conversion devices such as a multi-cell battery pack or a fuel cell. The presence of high-voltage buses creates a need for monitoring of the electrical isolation of each bus with respect to the electrically conductive components of the vehicle chassis (ground).
Any leakage resistance present between a DC bus and chassis ground must be sufficiently large. In typical leakage resistance detection systems, there is an assumption that leakage resistance will be between one of the positive or negative DC buses and chassis ground. A typical leakage detector circuit operates by periodically connecting one bus at a time to chassis ground through a current-limiting resistance, and using the resulting current flow to calculate the leakage resistance between the opposite bus and ground. The calculated leakage resistance divided by battery voltage characterizes the electrical isolation.
Leakage resistance detection systems typically supply a current from one bus through a known resistance to the leakage resistance between the ground and the other bus. Such systems might miss a potential balanced component of leakage resistance from both buses to ground that can sometimes result in the mischaracterization of the electrical isolation because of a potential discrepancy in the derived leakage resistance values. More specifically, a resistance may exist between one bus and chassis ground as well as a resistance having a component of equal value between the other bus and chassis ground. These resistances, both being equal in value, are hereafter referred to as symmetrical or balanced leakage resistance. A resistance on one bus to chassis without a matching value on the other bus to chassis is hereafter referred to as non-symmetrical or unbalanced leakage resistance. The additional current flow through the balanced leakage resistance may cause prior art detection systems to overestimate the composite balanced and unbalanced resistance which exists between one bus and chassis ground. Estimating this latter composite resistance is desirable in order to more accurately determine the electrical isolation.
Co-pending application U.S. Ser. No. 14/504,588, entitled “Bus Leakage Resistance Estimation for Electrical Isolation Testing and Diagnostics,” filed Oct. 2, 2014, which is incorporated herein by reference in its entirety, discloses methods and apparatus for identifying the balanced and unbalanced leakage resistances. Leakage currents are measured for the positive and negative buses. A correction factor based on a ratio of the currents is obtained in order to separate the balanced and unbalanced leakage resistance components.
The foregoing leakage resistance estimations depend on both voltage and current measurements from each of the detection circuits for the two respective buses. It would be desirable to reduce the number of measurements required and to reduce the computations needed to derive the unbalanced and/or balanced bus leakage resistances.